#define NULL 0

char* find_char(char const* source, char const* chars) {
    char* cp;
    if(source != NULL && chars != NULL) {
        for(; *source != '\0', source++) {
            for(cp = chars; *cp != '\0'; cp++) {
                if(*source == *cp)
                    return source;
            }
        }
    }
    return NULL;
}